This is a request for a research grant in support of studies of the role of cell-mediated immunity in chickens infected with Marek's disease virus. The objective of this research is to provide in a unique tumor model system further evidence that the cell-mediated immune response has an important function in resistance of the host to neoplastic disease. The investigations will focus on the identification of virus- specific antigens important for cell-mediated reactions, the characterization of the cell-mediated responses including possible interactions with humoral antibodies, the modifications of the pathogenesis in Marek's disease caused by cell-mediated immune responses, and on the possibility of enhancement of immune responses beneficial to the host. Chickens genetically resistant and susceptible to Marek's disease and of known immune status, and virus strains of different pathogenicity will be used in the studies. The detection and quantitation of cell- mediated immunity will be based upon techniques for cytotoxic assays developed in this laboratory.